County health officials advised the 80,000-student district to close after another four probable cases were found among students. Parents were advised to keep children living in the district away from child-care centers, movie theaters, restaurants or other public places.

The Centers for Disease Control in Atlanta -- at midday -- said the breakdown of confirmed swine flu cases was: New York, 50; Texas, 26 with one death; California, 14; South Carolina 10; Massachusetts and Kansas, two each, and Arizona, Indiana, Michigan and Ohio, one each.

The World Health Organization, based in Geneva, Switzerland, reported on its Web site that as of late Wednesday nine countries officially reported 148 cases of swine flu -- or A/H1N1.

WHO also raised the pandemic alert level from Phase 4 to Phase 5, the second-highest alert.

"All countries should immediately activate their pandemic preparedness plans," WHO Director General Dr Margaret Chan said. "At this stage, effective and essential measures include heightened surveillance, early detection and treatment of cases, and infection control in all health facilities."

The CDC's acting director, Dr. Richard Besser, said raising the alert level was good because it provides "a wake-up call" to countries, telling officials "it's time to pull out your pandemic plans ... and think about what you're going to do."

China's Health Ministry said researchers have developed an effective means for instant diagnosis of the virus, China's state-run Xinhua reported. Health Minister Chen Zhu said the new detection method will be installed at the country's center for disease control and prevention offices.